A liquid crystal display (hereinafter LCD) is used in several products since it is light and thin and has the low power consumption. As the products, for example, in big size displays, there are a note book personal computer, a TV receiver with large screen and the like. Further, as the middle or small size product, there are a mobile phone, a personal digital assistant (PDA) and the like. Especially in recent years, the LCD is often used in the big type display.
In the LCD, a polymer film is used, since it is reasonable and excellent in flexibility. The polymer film is used as a protective film for a polarizing filter in the LCD and as a support of a retardation film for widening a view angle of the LCD. The film used in the LCD is called an optically functional film. Further, researchers develop rapidly the researches for a VA (vertically aligned) type and OCB (optically compensated bend) type that are produced by widening the view angle of a TN (twisted nematic) type. Also to these LCD of indication mode, a large number of the optically functional film is used. The optically functional film is produced by a solution casting method or a melt extrusion method the same as the polymer film for general use (for example, Electronic Display Technology 2004, Kogyo Chosakai Publishinig. Co., LTD., June, 2004).
In the solution casting method, a polymer such as a cellulose triacetate (hereinafter TAC) is dissolved to a mixture solvent whose main solvent is dichloromethane and methyl acetate, such that a polymer solution (hereinafter casting dope) may be obtained. The casting dope is cast from a casting die onto a support so as to form a casting film, while the discharged dope between the casting die and the support forms a bead. When the casting film has a self-supporting property, the casting film is peeled as a wet film, and the wet film is dried to be a film. Then the film is to be wound up. (cf: Japan Institute of Invention and Innovation (JIII) Journal of Technical Disclosure No. 2001-1745).
By the way, it is necessary for the optical functional film, especially the protective film of the polarizing filter to provide with not only high transparency and high strength but also high heat resistance. If the heat resistance of the protective film is not enough, the shrinkage and the deterioration of the protective film easily occurs under the high temperature and high humidity, and otherwise the deterioration and the like of the adhesive agent layer between the protective film and a glass plate of a liquid crystal cell. The shrinkage, the deterioration and the like causes the peeling of the protective film from the glass plate.
Therefore, there is a method in which other polymer is used than TAC in order to increase the moisture resistance and the heat resistance of the film to be produced by the solution casting method. For example, for the acylation of the cellulose, as described in Japanese Patent Laid Open Publication No. 2001-188128, the acylations are made by acetyl group (—CO—CH3) and propionyl group (CO—C2H5), namely the esterification is made with acetic acid and propanoic acid, such that cellulose acylate propionate (hereinafter CAP) may be produced, and then the CAP is used as a raw material of the film.
The melt-extrusion method is performed as follows. For example, tips of polyethylene telephthalate (hereinafter PET) are produced from telephthalic acid and ethylene glycol. Then the PET tips are heated to melt, and the melt PET is extruded from an extruder onto a cooled drum as a support, so as to form a film. Note that other polyester films such as polyethylene naphthalate, cellulose film and the like are produced by the same method. Further, in the melt-extrusion method, in order to obtain the film of the objected optical properties, the stretch in a widthwise direction is made, and further the relaxation in a longitudinal direction is made. (cf: Japan Institute of Invention and Innovation (JIII) Journal of Technical Disclosure No. 2001-1745). If the stretch and the relaxation of the film are combined, the birefringency of the film becomes adjustable. The film having the birefringency is used as a retardation film of the LCD.
By performing the stretch in the widthwise direction and the relaxation in the longitudinal direction, the orientation of the polymer molecules is adjusted. Thus an in-plane retardation or a thickness retardation of the film is controlled. However, processes after the stretching process or the relaxation process cause the reorientation of the polymer molecules. Therefore, the film having the objected optical properties is hardly obtained.
High optical properties are required for the optically functional film to be used in the LCD of VA type for that a high speed response and a wide view angle are necessary. According to the LCD of the VA type, in order to realize the high speed response, a cell gap between glass plates for sandwiching liquid crystal molecules is made smaller. In this case, the in-plane retardation (Re) of the optical functional film becomes larger, and therefore there is a merit in view of an optical compensation. However, the thickness retardation (Rth) also becomes larger at the almost same ratio as the in-plane retardation (Re). Thus the thickness retardation is too large in comparison to the cell gap, which causes the worse optical properties of the LCD.
An object of the present invention is to provide a producing method of a polymer film in a batch process which has a predetermined optical properties.
Another object of the present invention is to provide a producing method of a polymer film in which an in-plane retardation is larger and a thickness retardation is lower than the prior art.